Wastewater streams containing ammonia are provided by a number of sources, including, but not limited to, industrial wastewater streams and municipal wastewater streams. The use of the term ammonia herein is intended to include both ionized ammonia (ammonium cation) and non-ionized ammonia. The equilibrium between ionized and non-ionized ammonia is affected by the composition and pH of the wastewater stream and its environment including, but not limited to, temperature, pressure, and head space gas composition, as is well known in the art.
Due to environmental and public health concerns, removal of ammonia from wastewater streams is desired. One such process for removal of ammonia is by metabolic oxidation using, typically in the presence of Nitrosomonas bacteria, to produce nitrate anion which in turn can be removed by microbial denitrification to dinitrogen gas. On a stoichiometric basis, 4½ atoms of oxygen are required for conversion of ammonia to nitrate anion and water. Accordingly, the capital and operating expenses to supply air or other oxygen-containing gas to an ammonia oxidation unit operation can represent a significant expense. Additionally the microbial denitrification process requires the presence of an electron donor, usually organic carbon, for the reduction to dinitrogen gas. Alternatively the ammonia can be oxidized to nitrite anion (nitritation) thus reducing the stoichiometric oxygen requirement. The nitrite anion can then be removed by microbial denitrification to dinitrogen gas.
A further additional proposal is an anoxic metabolic process for ammonium oxidation with nitrite anion as an electron acceptor. This process is sometimes referred to as the anammox process. As can be well appreciated, nearly a quarter of the oxygen required to convert ammonia to nitrate anion can be saved when the ammonia is converted to nitrite anion. This savings in oxygen can sometimes be well over 50 percent when the anammox process is used. The anammox process can be conducted in two separate bioreactors, the first for forming nitrite anion, and the second for the anoxic oxidation of ammonia. Proposals also exist to conduct the anammox process in a single reactor using biofilm or sludge granules containing a region of ammonia oxidizing bacteria and a region of bacteria capable of effecting the anoxic oxidation. This single step anammox process can be challenging in that the ammonia oxidizing bacteria and bacteria capable of effecting the anoxic oxidation of ammonia need to be maintained in balance. Consequently the two step process provides operational advantages.
One challenge that has faced the industry is that the nitritation process, alone or in combination with the anammox process results in the production of nitric oxide and nitrous oxide. These compounds have limited solubility in water and thus are readily exhausted to the atmosphere. The greenhouse effect of nitrous oxide is reported to be about three hundred times that of carbon dioxide. The large volumes of wastewater being processed in a municipal wastewater plant can result in a sizable point source of nitrous oxide emission.
The literature has reported a wide range of productions of nitric oxide and nitrous oxide off gases from wastewater treatment, but there is uniform agreement that the production of nitric oxide is very small in comparison to that of nitrous oxide. Kampschrer, et al., in “Emission of nitrous oxide and nitric oxide from a full-scale single-stage nitritation-anammox reactor”, Water Sci. Technol., 60(12), 2009, pages 3211-3217, conclude from their study that the omission of nitrous oxide during normal operation is about 1.23 percent of the nitrogen load to the reactor (1.67 percent of the nitrogen removed). They suggest that implementation of process control parameters may be able to minimize the amount of nitric oxide and nitrous oxide production. The authors indicate that the production of nitrous oxide is increased with increasing nitrite anion concentration. Thus two step nitritation/anoxic oxidation processes would be expected to be more prone to produce nitrous oxide than would single step processes.
Another challenge is minimizing the coproduction of nitrate anion during nitrification, especially using nitrifying microorganisms that are capable of converting nitrite anion to nitrate anion. The coproduction of nitrate anion is particularly problematic for the anammox process in that the nitrate anion is not an effective electron acceptor. If significant co-production of nitrate anion occurs, an additional unit operation may be required to remove nitrate anion before the effluent from the anammox process can be discharged to the environment.
Accordingly processes are sought for the nitritation of ammonia that minimize the emission of nitrous oxide and/or the coproduction of nitrate anion, especially for such processes where the minimization of the emission of nitrous oxide or coproduction of nitrate anion does not require difficult process controls to maintain tight tolerances of, for example, pH and dissolved oxygen. Moreover, processes are sought that enable a high conversion of ammonia to nitrite anion, e.g., at least about 40 atomic percent, and sometimes at least about 70, and preferably at least about 95, atomic percent, of the ammonia nitrogen being converted to nitrite anion, while minimizing nitrous oxide emissions and/or the coproduction of nitrate anion. Such processes would be particularly useful for use with the anammox process.